Method for reducing oxygen component and carbon component in fluoride

ABSTRACT

To provide a method for reducing oxygen and carbon components in high-purity fluoride such as calcium fluoride and barium fluoride (high-purity fluoride is not limited to calcium fluoride and barium fluoride), or in rare-earth fluoride such as ytterbium fluoride and cerium fluoride used for an optical fiber, a coating material, and so on.  
     It is characterized in that the oxygen and carbon components contained in the high-purity fluoride are reduced by a fluorine gas treatment.

TECHNICAL FIELD

[0001] The present invention relates to a method for reducing an oxygen component and a carbon component in fluoride.

BACKGROUND ART

[0002] In the lithographic process in the manufacture of a semiconductor, it is applied on an optical-purpose material such as a raw material of an optical system that constitutes a stepper device provided as an exposure device, such as a projective lens or an illumination lens, or a raw material as an optical fiber material or a coating material.

[0003] For instance, in the case of a lens material, an ArF excimer laser (193 nm) which is an exposure source of light having a short wavelength and an F2 excimer laser (157 nm) having a more shorter wavelength, has been used in connection with a high integration degree. However, since a high-purity silica glass provided as a lens being typically used heretofore has a poor transmittance, it cannot be used. Thus, fluorides such as calcium fluoride and barium fluoride, which have good transmittance properties even at short wavelength and strong laser durability, have been considered as being useful. Whereas, a high purity monocrystal is absolutely essential for the manufacture of a fluoride lens made of calcium fluoride, barium fluoride, or the like. It has been known that various kinds of problems may be caused when an excess amount of oxygen or carbon is included as impurities in the raw material.

[0004] The oxygen component may be incorporated into fluoride as moisture is remained at the time of the manufacture, or oxide is generated by the process of sintering, so that finally, at the time of being formed into a lens, (1) a reduction in transmittance caused by oxygen, (2) laser durability, (3) coloring, and so on may be influenced. Thus, it has been said that it would make an influence on the transmittance of light for which low concentration has been desired. Therefore, the low concentration has been desired.

[0005] The carbon component is caused from a crucible material at the time of sintering, and typically an alumina crucible or a carbon crucible is used. When the alumina crucible is used, contamination with aluminum may be occurred. In general, therefore, the carbon crucible is used. However, carbons as a crucible material may remain in fluoride. In the process of manufacturing the fluoride, furthermore, unreacted carbons from carbonate as a raw material generally used at the time of manufacturing fluoride may remain in the product. These remaining carbons in the fluoride product are said to be incorporated into a monocrystal at the time of manufacturing a monocrystal (1), and exert an adverse effect on coring or the like (2).

[0006] Actually, at the time of manufacturing the monocrystal, the oxygen component is generally used as a scavenger, while lead fluoride or zinc fluoride is used. For instance, the lead fluoride performs a deoxidization process of the following reaction (A) is performed.

CaO+PbF₂→CaF₂+PbO↑  (A)

[0007] The deoxidization process of the above (A) permits a decrease in the amount of oxygen. However, as the scavenger is added as a second component, contamination generates therefrom to make the formation of a pure monocrystal impossible. For minimizing the addition amount of the scavenger as much as possible, there is a need of fluoride containing a smaller amount of the oxygen component in the raw material.

[0008] An object of the present invention is to provide a method for reducing an oxygen component and a carbon component in fluoride, which is capable of lowering the amounts of oxygen and carbon in the fluoride by a simple process.

DISCLOSURE OF THE INVENTION

[0009] The present invention relates to a method for reducing an oxygen component and a carbon component in fluoride, characterized by performing a fluorine gas treatment for bringing the fluoride into a contact with gas containing fluorine gas.

[0010] Operation and Embodiment

[0011] The present invention has found out a method capable of reducing carbon in addition to oxygen which is an impurity contained in a product by subjecting calcium fluoride, barium fluoride, or the like provided as for example a lens raw material to a fluoride gas treatment as a secondary treatment. It is found that such a method has effects for reducing oxygen and carbon by simply introducing fluorine gas as a secondary treatment on a conventional product and mixing together. The fluorine gas is a gas having a strong oxidizing ability. It is thought that an oxygen component and a carbon component in the impurities may be reduced by the following reactions. However, at the present, a distinct reaction mechanism has not been revealed.

O+F₂→OF₂↑  (B)

C+F₂→CF₄↑  (C)

[0012] For the conditions of the secondary treatment with fluorine gas, but the effects of low oxygen and low carbon are obtainable without depending on the concentration of fluorine, preferably 5 to 50% fluoride gas content (inert gas such as nitrogen or argon gas is provided for diluting gas) is advantageous in terms of industry and handling. Regarding the processing temperature as a matter of course, in addition, an active energy decreases as the temperature increases. It is advantageous for the reaction. In the case of processing under the high temperature, there is a need of using a reactor made of a metal material. For reducing the generation of contamination from the reactor, the temperatures under which a polymer material such as a fluorocarbon resin can be reduced is preferable in terms of handling. Furthermore, at a room temperature, the effects of low oxygen and low carbon were obtained.

[0013] By the way, the concentration of the fluorine gas may be 100%, or may be diluted with inert gas. At this time, the concentration of fluorine gas may be preferably 5 to 50%.

[0014] The inert gas available for the dilution includes, for example, nitrogen, argon, and neon. It is preferable that the inert gas may be of a high purity with an impurity (e.g., moisture or oxygen) concentration of 10 ppb or less.

[0015] The fluoride may be preferably a rare-earth fluoride. Specifically, it is one or more selected from calcium fluoride, barium fluoride, ytterbium fluoride, cerium fluoride, and ytterbium fluoride.

[0016] A method for reducing oxygen content and carbon content in the fluoride described in one of claims 1 to 4, characterized in that.

[0017] A fluoride gas treatment is preferably performed at a room temperature to 350° C., more preferably 200 to 300° C.

[0018] Fluoride is favorably used as an optical material to be provided as a raw material of an optical system (e.g., projector lens or illumination lens) that constitutes an exposure device to be used in the process of lithography, an optical fiber material, a coating material, or the like.

[0019] The external shape of fluoride is not specifically limited. Preferably, however, the fluoride is shaped in the form of powders, granules, or particles.

[0020] The diameter thereof is preferably 1 mm or less, more preferably 1 to 500 μm, more preferably 10 to 200 μm, and still more preferably 10 to 200 μm.

[0021] The duration of fluorination is, preferably from 30 seconds to 1 hour, depending on the temperature. More it is preferably from 1 to 30 minutes, and still more preferably of 1 to 10 minutes because effects will be saturated even in a longer duration.

[0022] The loner duration will cause the saturation of effects.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0023] A purification treatment was performed by placing 300 g of high-purity calcium in a 5L-PFA container and sufficiently mixing the powders and gas while feeding the flow of gas based on each of 5, 10, 25, and 50% of fluorine gas/N₂ for 5 minutes at room temperature. The treated product was then subjected to analyses for oxygen and carbon using an oxygen analyzer and a carbon analyzer, respectively (Table 1). TABLE 1 The results of Example 1 (Secondary treatment at room temp.) Oxygen (ppm) Carbon (ppm) F₂ concentration (%) 5 10 25 50 5 10 25 50 Before F₂ gas treatment 140 145 145 140 30 35 35 30 After F₂ gas treatment 80 85 80 80 <5 <5 <5 <5

[0024] From the results described above, reductions of oxygen and carbon could be confirmed by means of the secondary treatment without depending on the respective 5, 10, 25, and 50% fluorine gas concentrations. Furthermore, the effects could be found even at room temperature, and this finding would have large industrial merits.

EXAMPLE 2

[0025] In the same way as that of Example 1, a purification treatment was performed by placing 300 g of high-purity calcium in a 5L-PFA container, maintaining a reaction container at 100° C., and sufficiently mixing the powders and gas while feeding the flow of gas based on each of 5, 10, 25, and 50% of fluorine gas/N₂ gas for 5 minutes, followed by oxygen and carbon analyses, respectively (Table 2). TABLE 2 The results of Example 2 (Secondary treatment at 100° C.) Oxygen (ppm) Carbon (ppm) F₂ concentration (%) 5 10 25 50 5 10 25 50 Before F₂ gas treatment 140 145 145 140 30 35 35 30 After F₂ gas treatment 80 85 80 80 <5 <5 <5 <5

[0026] From the results described above, reductions of oxygen and carbon were confirmed by means of the respective 5, 10, 25, and 50% fluorine gas concentration treatments. Furthermore, the reduction effects were found when the treatment temperature was increased.

EXAMPLE 3

[0027] The same experiment as shown in Example 1 was performed using rare-earth fluoride such as ytterbium fluoride or cerium fluoride to be used as an optical fiber material or a coating material, for example, ytterbium fluoride. A purification treatment was performed by placing 300 g of ytterbium fluoride in a 5L-PFA container and mixing the powders and gas while feeding the flow of gas based on each of 5, 10, 25, and 50% of fluorine gas/N₂ gas for 5 minutes at room temperature.

[0028] The treated product was then subjected to analyses for oxygen and carbon using an oxygen analyzer and a carbon analyzer, respectively (Table 3). TABLE 3 The results of Example 1 (Secondary treatment at room temp.) Oxygen (ppm) Carbon (ppm) F₂ concentration (%) 5 10 25 50 5 10 25 50 Before F₂ gas treatment 800 800 850 830 30 35 35 30 After F₂ gas treatment 150 130 180 150 <5 <5 <5 <5

[0029] From the results described above, the effects of low oxygen and low carbon were obtained just as in the case of calcium fluoride in Example 1. Likewise, furthermore, the effects of low oxygen and low carbon were obtained by increasing the treatment temperature of Example 2.

EXAMPLE 4

[0030] A secondary treatment was performed by placing 300 g of a high-purity calcium fluoride of Example 1 in a heat-resisting metallic container, for example, a nickel container (a stainless container may also be used) and introducing 100% fluorine gas at 300° C. for 5 minutes.

[0031] The treated product was then subjected to analyses for oxygen and carbon using an oxygen analyzer and a carbon analyzer, respectively (Table 4). TABLE 4 The results of Example 4 (Secondary treatment at 300%) Oxygen (ppm) Carbon (ppm) Before F₂ gas treatment 145 130 After F₂ gas treatment 70 <5

[0032] From the results described above, using 100% fluorine gas, and also in the treatment at a high temperature of 300° C., the effects of low oxygen and low carbon were confirmed.

INDUSTRIAL APPLICABILITY

[0033] The present invention allows: the refinement of oxygen and carbon, for example, which are impurities in a high-purity calcium fluoride known as a typical raw material used for a stepper lens for an ArF or F2 excimer-laser in the photolithographic process during the manufacture of a semiconductor; a reduction in the adding amount of a scavenger in the deoxidization during the manufacture of a monocrystal; and a reduction in various kinds of problems. Furthermore, in recent years, thin-film technologies have been progressed. There is a need of a low-oxygen high-purity fluoride, and there is a possibility of its availabilities in many other fields. 

1. A method for reducing an oxygen component and a carbon component in fluoride, characterized by performing a fluorine gas treatment in which fluoride is brought into contact with gas that contains fluorine gas.
 2. A method for reducing an oxygen component and a carbon component in fluoride as described in claim 1, wherein the concentration of fluorine gas in the gas containing fluorine gas is 5 to 50%.
 3. A method for reducing an oxygen component and a carbon component in fluoride as described in claim 1, wherein the gas that contains the fluorine gas is gas prepared by diluting fluorine gas with inert gas.
 4. A method for reducing an oxygen component and a carbon component in fluoride as described in one of claims 1 to 3, wherein the fluoride is rare-earth fluoride.
 5. A method for reducing an oxygen component and a carbon component in fluoride as described in one of claims 1 to 4, wherein the fluoride is selected from calcium fluoride, barium fluoride, ytterbium fluoride, and cerium fluoride.
 6. A method for reducing an oxygen component and a carbon component in fluoride as described in one of claims 1 to 5, wherein the fluorine gas treatment is performed at room temperature to 350° C.
 7. A method for reducing an oxygen component and a carbon component in fluoride as described in claim 6, wherein the fluorine gas treatment is performed at 200 to 300° C.
 8. A method for reducing an oxygen component and a carbon component in fluoride as described in one of claims 1 to 7, wherein the fluoride is fluoride used for an optical material.
 9. A method for reducing an oxygen component and a carbon component in fluoride as described in one of claims 1 to 8, wherein the fluoride is in the form of powders, granules, or particles.
 10. A method for reducing an oxygen component and a carbon component in fluoride as described in claim 9, wherein the diameter of the fluoride is 1 mm or less.
 11. A method for reducing an oxygen component and a carbon component in fluoride as described in claimed in claim 10, wherein the diameter of the fluoride is 10 to 200 μm.
 12. A method for reducing an oxygen component and a carbon component in fluoride as described in claimed in one of claims 1 to 11, wherein the fluorination time is 30 second to 1 hour.
 13. A method for reducing an oxygen component and a carbon component in fluoride as described in claim 12, where in the fluorination time is 1 to 10 minutes. 